Is money earned from delivering pizza orders containing meat of doubtful permissibility lawful or unlawful, and how can unlawful money be disposed of if that is the case?

As long as the matter is merely a doubt and a possibility that the meat is not ritually slaughtered, the default is that it is permissible, and your ownership is not unlawful, nor are you required to dispose of it. The default for meat imported from the People of the Book or from Muslim countries is permissibility, until proven otherwise. This is supported by the hadith of Aisha, may Allah be pleased with her: "Indeed, some people said: 'O Messenger of Allah, some people bring us meat, and we do not know whether they mentioned Allah's name over it or not?' So the Prophet, peace and blessings be upon him, said: 'You mention Allah's name and eat.'" This hadith indicates that if an action is performed by its people, it is not obligatory to inquire about its manner.